1. Field
Example embodiments relate to methods and apparatuses for operating and repairing nuclear reactors. Additionally, example embodiments relate to methods and apparatuses for operating and repairing nuclear reactors that include one or more submerged lines welded to one or more support brackets.
2. Description of Related Art
FIG. 1 is a sectional view, with parts cut away, of reactor pressure vessel (“RPV”) 100 in a related art BWR. During operation of the BWR, coolant water circulating inside RPV 100 is heated by nuclear fission produced in core 102. Feedwater is admitted into RPV 100 via feedwater inlet 104 and feedwater sparger 106 (a ring-shaped pipe that includes apertures for circumferentially distributing the feedwater inside RPV 100). The feedwater from feedwater sparger 106 flows down through downcomer annulus 108 (an annular region between RPV 100 and core shroud 110).
Core shroud 110 is a stainless steel cylinder that surrounds core 102. Core 102 includes a multiplicity of fuel bundle assemblies 112 (two 2×2 arrays, for example, are shown in FIG. 1). Each array of fuel bundle assemblies 112 is supported at or near its top by top guide 114 and at or near its bottom by core plate 116. Top guide 114 provides lateral support for the top of fuel bundle assemblies 112 and maintains correct fuel-channel spacing to permit control rod insertion.
The coolant water flows downward through downcomer annulus 108 and into core lower plenum 118. The coolant water in core lower plenum 118 in turn flows up through core 102. The coolant water enters fuel assemblies 112, wherein a boiling boundary layer is established. A mixture of water and steam exits core 102 and enters core upper plenum 120 under shroud head 122. Core upper plenum 120 provides standoff between the steam-water mixture exiting core 102 and entering standpipes 124. Standpipes 124 are disposed atop shroud head 122 and in fluid communication with core upper plenum 120.
The steam-water mixture flows through standpipes 124 and enters steam separators 126 (which may be, for example, of the axial-flow, centrifugal type). Steam separators 126 substantially separate the steam-water mixture into liquid water and steam. The separated liquid water mixes with feedwater in mixing plenum 128. This mixture then returns to core 102 via downcomer annulus 108. The separated steam passes through steam dryers 130 and enters steam dome 132. The dried steam is withdrawn from RPV 100 via steam outlet 134 for use in turbines and other equipment (not shown).
The BWR also includes a coolant recirculation system that provides the forced convection flow through core 102 necessary to attain the required power density. A portion of the water is sucked from the lower end of downcomer annulus 108 via recirculation water outlet 136 and forced by a centrifugal recirculation pump (not shown) into a plurality of jet pump assemblies 138 (only one of which is shown) via recirculation water inlets 140. Jet pump assemblies 138 are circumferentially distributed around core shroud 110 and provide the required reactor core flow.
As shown in FIG. 1, a related art jet pump assembly 138 includes a pair of inlet mixers 142. A typical BWR includes 16 to 24 inlet mixers 142. Each inlet mixer 142 has an elbow 144 welded to it that receives water from a recirculation pump (not shown) via inlet riser 146. An example inlet mixer 142 includes a set of five nozzles circumferentially distributed at equal angles about the axis of inlet mixer 142. Each nozzle is tapered radially inwardly at its outlet. Jet pump assembly 138 is energized by these convergent nozzles. Five secondary inlet openings are radially outside of the nozzle exits. Therefore, as jets of water exit the nozzles, water from downcomer annulus 108 is drawn into inlet mixer 142 via the secondary inlet openings, where it is mixed with coolant water from the recirculation pump. The coolant water then flows into jet pump assembly 138.
FIG. 2 is a sectional view, with parts cut away and/or in silhouette, of the interior of RPV 100 in a related art BWR. Each jet pump assembly 138 has a sensing line 200 that is in fluid communication with a plurality of pressure taps at the top of diffuser 148 and with instrumentation (not shown) located outside RPV 100. Sensing lines 200 allow the core flow to be measured and monitored. The flow through and outside jet pump assemblies 138 includes pressure fluctuations from various sources in the nuclear reactor. These pressure fluctuations may have frequencies close to one or more natural vibration modes of sensing lines 200. These vibration modes depend on the spacing and stiffness of sensing lines 200 and support brackets 202 that attach sensing lines 200 to one or more of jet pump assemblies 138. When an excitation frequency happens to be too close to matching the natural frequency of one or more sensing lines 200 at some particular location, vibration of sensing lines 200 may exert loads on sensing lines 200 and/or support brackets 202 that can cause cyclic fatigue cracking and/or failure of sensing lines 200 and/or support brackets 202. This phenomenon may result in loss of the indication of core flow which, if it occurs at enough locations, may require plant shutdown.
Sensing lines 200 also may be subjected to damage caused by objects (i.e., head bolts for core shroud 110) falling on sensing lines 200 during maintenance, repair, and/or other procedures. This may be particularly true for sensing lines 200 oriented in a substantially horizontal direction, such as sensing lines 200 routed from jet pump assemblies 138 to penetration nozzle 204 through wall 206 of RPV 100.
Various solutions to the problems of jet pump sensing line failure, repair, and replacement have been proposed, as discussed, for example, in U.S. Pat. Nos. 5,615,239 (“the '239 patent”), 5,752,807 (“the '807 patent”), 6,163,588 (“the '588 patent”), 6,233,301 B1 (“the '301 patent”), and 6,435,839 B1 (“the '839 patent”). The disclosures of the '239 patent, the '807 patent, the '588 patent, the '301 patent, and the '839 patent are incorporated in this application by reference. However, these various solutions do not include methods or apparatuses for operating and repairing nuclear reactors that include one or more submerged lines welded to one or more support brackets, wherein damage to one or more of the submerged lines can be repaired without welding.